The Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross linking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
Examples of light sensitive printing plates include UV-sensitive pre-sensitized plates which may be based on a positive or negative working mechanism. Typical examples of positive working plates have an imaging layer comprising an o-naphtoquinonediazide compound (NQD) and an alkali soluble resin. The negative working pre-sensitized plates preferably comprise a diazonium salt, a diazonium resin or an aryldiazosulfonate homo- or copolymer. Examples of diazo resins include condensation products of an aromatic diazonium salt as described in for example DE 1 214 086. Light sensitive printing plates based on a photo-polymerisation reaction typically contain a coating comprising a photocurable composition comprising a free radical initiator, a polymerizable compound and a polymeric binder.
In general, aluminum substrates are used as supports for lithographic printing plates. The use of aluminum substrates as supports requires that they undergo several treatments such as for example graining and anodizing. Lithographic supports are roughened or grained to improve the adhesion of an imaging layer to the support and anodizing may be carried out to improve the abrasion resistance and water retention or wetting characteristics of the non-image areas of the support.
The aluminum support is typically roughened or grained by a process including:                (i) a mechanical roughening step: for example scraping mechanically the aluminum support; and/or        (ii) an electrochemical roughening step: electrolyzing the surface of the aluminum support in an electrolyte solution using the support as an electrode and for example graphite as counter electrode.        
By varying the type and/or concentration of the electrolyte solution and the applied voltage in the electrochemical roughening step, different type of grains can be obtained. Usually an alternating current such as a sine wave current, a trapezoidal wave current, or a rectangular wave current is applied while the aluminum support is immersed in an acidic electrolyte solution. Thus, the support is alternately subjected to a positive and a negative voltage at the entrance of an electrolysis cell. When the positive voltage is applied, a cathodic reaction occurs on the surface of the aluminum; when the negative voltage is applied, an anodic reaction occurs. During the cathodic reaction, an oxide layer is formed and when the anodic reaction occurs, the oxide layer is resolved into the acidic electrolyte to form honeycomb-shaped pits on the surface of the substrate. The surface of an unroughened aluminum printing plate support behaves in a nonlinear fashion when an electric current is applied to it due to the presence of for example aluminum oxide at the surface. Therefore, the current density is not only dependent on the applied voltage but additionally on the nature of the surface. The anodic started current tends to start a graining pattern that looks different from a cathodic started current graining, in that sense that more local larger pits are formed resulting in an inhomogeneous graining pattern. The graining pattern in a region where the cathodic current started, is much more homogeneously distributed over the whole surface. This difference in behaviour between the anodic and cathodic started areas in the graining process is especially observed at low current densities, typically during the first 100 C/dm2. Above 100 C/dm2, a homogeneous graining pattern will be superimposed on the inhomogeneous graining already present at that moment. This results in an optical difference between the anodic and the cathodic started areas and the human eye is able to percept this as so-called chattermarks. Chattermarks appear as a Moiré-pattern on the surface of a grained aluminum support. The tendency for the appearance of chattermarks on the aluminum surface is higher when a high current density is applied at the beginning of the electrochemical roughening. Many attempts have been carried out in the prior art to avoid these surface defects by modifying the graining conditions.
DE 38 42 454 C2 discloses a method wherein the surface of the printing plate substrate is provided with an additional layer whereby non-uniformities in the material that essentially cause spots are compensated for.
U.S. Pat. No. 6,423,206 discloses a method for electrochemically roughening the surface of the substrate in an aqueous electrolyte bath by the application of an alternating or three-phase current to special shaped electrodes opposite to the substrate, while the substrate is passed continuously through the electrolyte bath.
DE 39 10 450 C2 describes a method for producing a printing plate substrate in which the surface of said substrate is roughened electrochemically in an acidic electrolyte solution using an alternating current at a frequency of 80-100 Hz, and in which the ratio of anode time to period time is from 0.25 to 0.20.
EP 0 585 586 discloses a method wherein a constant imposition of equal-sized positive and negative half-waves of the alternating current is applied to the surface of a printing plate substrate.
U.S. Pat. No. 4,919,774 discloses a method of graining a metal web in an electrolytic liquid using an alternating wave-form current and whereby the ratio of the current value contributing to an anode and to a cathode reaction occurring on the surface of said metal web is controlled by shunting a part of the current value as a direct current into an auxiliary anode electrode provided separately from a pair of main electrodes.
U.S. Pat. No. 6,780,305 discloses a method for making an aluminum printing plate support, which can be produced from recycled aluminum, scrapped aluminum and regenerated aluminum, comprising a surface roughening step including (1) a pre-electrolytic surface roughening in an aqueous hydrochloric acid solution with an alternating or direct current applied thereto, (2) an alkali-etching step (3) a desmutting step with sulphuric acid and (4) an electrolytic surface-roughening step in an aqueous nitric acid solution with an alternating current being applied thereto.
US 2003\0105533 discloses an electrolysis apparatus wherein a support is conveyed at a high current density and a high conveyance velocity and which comprises a plurality of electrolysis cell arranged in series. An alternating current is applied so that the current density is lower at an electrolysis cell located at a most down-stream position compared to an electrolysis cell located upstream with respect to the conveyance direction.
JP 2004\243,633 discloses a method for making a printing plate support comprising an electrochemical surface roughening treatment using alternating current D ranging from 20 to 200 A/dm2, and a travel speed V through the electrolytic batch ranging from 70 to 160 m/min and wherein D≦122000×[V]−1.55.
EP 1,338,436 discloses a method for making an aluminum support comprising a graining step in a hydrochloric acid solution comprising chloride hexahydrate during which an alternating current is applied under the condition that the ratio of the quantity of electricity in the cathodic state Qc and the quantity of the electricity in the anodic state Qa is 0.9 to 1.0.
The methods and apparatuses proposed in the prior art for improving the surface characteristics of roughened aluminum are often complex and require a major expenditure for circuitry.